1. Technical Field
This invention relates to the technology of induction heating of ferrous metals, and more particularly to techniques for preventing scale on such inductively heated metals.
2. Discussion of the Prior Art
All induction heating of ferrous pieces, as practiced today, cause at least some scale to form on the heated surface of the part undergoing treatment. The surface of the part, at the temperature of inductive heating, is very reactive with oxygen contained in ambient air or quench water. The resulting oxide scale is undesirable because (i) it prevents the pieces from being finish machined prior to inductive heating, (ii) the scale must be subsequently removed such as by shot or grit blasting (for parts of a compact shape) for an extended period of time such as from 15 seconds to 2 minutes, or alternatively by machining the part (if not of a compact shape) to remove the scale while achieving a precise net shape. The use of shot or grit blasting is undesirable because it is not only expensive, time consuming and imprecise, but it is limited to surfaces that are not critical (those that can be changed metallurgically or dimensionally by the blasting).
Sometimes scale formation has been slightly retarded by use of an inert gas blown into the gap between the inductor and part to bathe the heated part. (See U.S. Pat. Nos. 2,679,466 and 2,799,604). Unfortunately, use of an inert gas fails to significantly prevent formation of scale since some oxygen always remains to combine with the iron at the elevated temperatures; the inert gas does not combine with the oxygen.
Some attempt has been made to enclose the entire inductor and part within a chamber to carry out the heating; in this arrangement an inert gas atmosphere is maintained in the chamber during heating to exclude the presence of oxygen at the heated part surface. Such apparatus reduces the productivity rate because of the need to open and close the chamber, fill and refill inert gas into the chamber as a result of the need for water quenching. But more importantly, closed chamber inductive heating delays hardening and increases the cost of facilitizing a plant to carry out such process, the cost being increased because of the need for more inductors to accomplish the same throughput. Such great increase in capital cost may be as much as five-fold.